/FRICTION
Block Format Keyword Specific contact friction between groups of parts or two parts. This friction definition overwrites the friction model defined in the contact interface for the defined set of interfaces.
This friction model is compatible with contact interfaces: TYPE7, TYPE11, TYPE19, TYPE24 and TYPE25.
Format
(1)  (2)  (3)  (4)  (5)  (6)  (7)  (8)  (9)  (10) 

/FRICTION/fric_ID/unit_ID  
friction_title  
I_{fric}  I_{filtr}  X_{freq}  I_{form} 
(1)  (2)  (3)  (4)  (5)  (6)  (7)  (8)  (9)  (10) 

C_{1}  C_{2}  C_{3}  C_{4}  C_{5}  
C_{0}  Fric  VIS_{F} 
(1)  (2)  (3)  (4)  (5)  (6)  (7)  (8)  (9)  (10) 

grpart_ID1  grpart_ID2  part_ID1  part_ID2  Idir  
C_{1}  C_{2}  C_{3}  C_{4}  C_{5}  
C_{6}  Fric  VIS_{F} 
(1)  (2)  (3)  (4)  (5)  (6)  (7)  (8)  (9)  (10) 

C_{1}  C_{2}  C_{3}  C_{4}  C_{5}  
C_{6}  Fric  VIS_{F} 
Definitions
Field  Contents  SI Unit Example 

fric_ID  Friction
identifier (Integer, maximum 10 digits) 

unit_ID  Unit Identifier (Integer, maximum 10 digits) 

friction_title  Friction model
title (Character, maximum 100 characters) 

I_{fric}  Friction formulation flag.
1
(Integer) 

I_{filtr}  Friction filtering flag.
5
(Integer) 

X_{freq}  Filtering
coefficient. This coefficient should have a value between 0 and 1. Default = 1.0 (Real) 

I_{form}  Friction penalty
formulation type. 6
(Integer) 

C_{1}  Friction law
coefficient. (Real) 

C_{2}  Friction law
coefficient. (Real) 

C_{3}  Friction law
coefficient. (Real) 

C_{4}  Friction law
coefficient. (Real) 

C_{5}  Friction law
coefficient. (Real) 

C_{6}  Friction law
coefficient. (Real) 

Fric  Coulomb
friction. (Real) 

VIS_{F}  Critical damping
coefficient on interface friction. 4 Default = 1.0 (Real) 

grpart_ID1  Part group identifier.
/GRPART for the first
set. (Integer) 

grpart_ID2  Part group identifier
/GRPART for the second
set. (Integer) 

part_ID1  Part identifier
1. Ignored if grpart_ID1 is defined. (Integer) 

part_ID2  Part identifier
2. Ignored if grpart_ID2 is defined. (Integer) 

Idir  Orthotropic friction flag
for a couple of parts.
(Integer) 
Example
#RADIOSS STARTER
#12345678910
/FRICTION/999
test no 1
# Ifric Ifiltr Xfreq Iform
0 0 0 2
# default friction for rest parts which not specifically defined below
# C1 C2 C3 C4 C5
0 0 0 0 0
# C6 Fric VisF
0 .2 0
#12345678910
#friction between part group ID 111 and ID 222
#GRpartID1 GRpartID2 PartID_1 PartID_2 Idir
111 222 0 0 0
# C1 C2 C3 C4 C5
0 0 0 0 0
# C6 Fric VisF
0 .1 0
#12345678910
#friction between part ID 1 and ID 3
#GRpartID1 GRpartID2 PartID_1 PartID_2 Idir
0 0 1 3 0
# C1 C2 C3 C4 C5
0 0 0 0 0
# C6 Fric VisF
0 .2 0
#12345678910
#friction between part ID 1 and ID 4; orthotropic direction considered
#GRpartID1 GRpartID2 PartID_1 PartID_2 Idir
0 0 1 4 1
# C1 C2 C3 C4 C5
0 0 0 0 0
# C6 Fric VisF
0 .4 0
# C1 C2 C3 C4 C5
0 0 0 0 0
# C6 Fric VisF
0 .2 0
#12345678910
#friction between part ID 1 and ID 5
#GRpartID1 GRpartID2 PartID_1 PartID_2 Idir
0 0 1 5 0
# C1 C2 C3 C4 C5
0 0 0 0 0
# C6 Fric VisF
0 .3 0
#12345678910
#12345678910
/INTER/TYPE7/2
New INTER 2
# Slav_id Mast_id Istf Ithe Igap Ibag Idel Icurv Iadm
9 10 0 0 2 0 1 0 0
# Fscalegap Gap_max Fpenmax
0 0 0.8
# Stmin Stmax %mesh_size dtmin Irem_gap Irem_i2
1 0 0 0 0 0
# Stfac Fric Gapmin Tstart Tstop
0 .35 2.1 0 0
# IBC Inacti VisS VisF Bumult
000 6 0 0 0
# Ifric Ifiltr Xfreq Iform sens_ID fct_IDf AscaleF fric_ID
0 0 0 2 0 0 0 0
/GRNOD/PART/9
INTER_group_9_of_SURF
4 5
/SURF/PART/10
INTER_group_10_of_PART
1
#12345678910
#12345678910
/PROP/SH_ORTH/11
PROPERTY FOR PART ID 1
# Ishell Ismstr Ish3n Idrill
24 0 0 1
# hm hf hr dm dn
0 0 0 .1 .1
# N Istrain Thick Ashear Ithick Iplas
5 0 1.0 0 1 1
# Vx Vy Vz Phi
1 0 1 45
#12345678910
Parts  I_{dir}  Friction Coefficient  

All parts not listed    0.2  
/GRPART/111  /GRPART/222  0: Isotropic  0.1  
part_ID1  part_ID3  0: Isotropic  0.2  
part_ID1  part_ID4  1: Orthotropic  Dir1 = 0.4  Dir2 = 0.2 
part_ID1  part_ID5  0: Isotropic  0.3 
In this example, the orthotripoc direction for friction between parts 1 and 4 is defined by the part 1 property /PROP/SH_ORTH/11 because part 1 is the main contact surface.
Comments
 The friction defined in /FRICTION overrides any friction defined in the contact interface.
 Default values listed in the first section are used for any parts whose friction is not specifically defined in the repeating section using grpart_ID1, grpart_ID2, part_ID1, and part_ID2.
 If friction between parts is defined more than one time in the model, the friction defined in the last position are used.
 The friction value
$\mu $
is defined.
 I_{fric} = 0 (Coulomb friction):
(1) $$\mu =\mathit{Fric}$$  I_{fric} = 1 (Generalized Viscous Friction law):
(2) $$\mu =\mathit{Fric}+{C}_{1}.p+{C}_{2}\cdot V+{C}_{3}.p\cdot V+{C}_{4}\cdot {p}^{2}+{C}_{5}\cdot {V}^{2}$$Where, $p$
 Pressure of the normal force on the main segment
 $V$
 Tangential velocity of the secondary node
 I_{fric} = 2 (Modified Darmstad law):
(3) $$\mu =\mathit{Fric}+{C}_{1}.{e}^{\left({C}_{2}V\right)}.{p}^{2}+{C}_{3}.{e}^{\left({C}_{4}V\right)}.p+{C}_{5}.{e}^{\left({C}_{6}V\right)}$$Where, $p$
 Pressure of the normal force on the main segment
 $V$
 Tangential velocity of the secondary node

 I_{fric} = 3 (Renard law):
(4) $$\mu ={C}_{1}+\left({C}_{3}{C}_{1}\right)\cdot \frac{V}{{C}_{5}}\cdot \left(2\frac{V}{{C}_{5}}\right)\hspace{0.17em}\text{if}\hspace{0.17em}V\in \left[0,{C}_{5}\right]$$(5) $$\begin{array}{l}\mu ={C}_{3}\left(({C}_{3}{C}_{4}\right)\cdot {\left(\frac{V{C}_{5}}{{C}_{6}{C}_{5}}\right)}^{2}\cdot \left(32\cdot \frac{V{C}_{5}}{{C}_{6}{C}_{5}}\right)\hspace{0.17em})\hspace{0.17em}\text{if}\hspace{0.17em}V\in [{C}_{5}{C}_{6}]\\ \end{array}$$(6) $$\mu ={C}_{2}\frac{1}{\frac{1}{{C}_{2}{C}_{4}}+{\left(V{C}_{6}\right)}^{2}}\text{\hspace{0.17em}}\text{\hspace{0.17em}}\text{if}\text{\hspace{0.17em}}\text{\hspace{0.17em}}V\ge {C}_{6}$$
Where,${C}_{1}={\mu}_{s}$ ${C}_{4}={\mu}_{\mathrm{min}}$ ${C}_{2}={\mu}_{d}$ ${C}_{5}={V}_{\mathit{cr}1}$ ${C}_{3}={\mu}_{\mathrm{max}}$ ${C}_{6}={V}_{cr2}$  First critical velocity ${V}_{cr1}={C}_{5}$ must be different to 0 ( ${C}_{5}\ne 0$ ).
 First critical velocity ${V}_{cr1}={C}_{5}$ must be less than the second critical velocity ${V}_{cr2}={C}_{6}\left({C}_{5}<{C}_{6}\right)$ .
 The static friction coefficient ${C}_{1}$ and the dynamic friction coefficient ${C}_{2}$ , must be less than the maximum friction ${C}_{3}$ ( ${C}_{1}\le {C}_{3}$ and ${C}_{2}\le {C}_{3}$ ).
 The minimum friction coefficient ${C}_{4}$ must be less than the static friction coefficient ${C}_{1}$ and the dynamic friction coefficient ${C}_{2}$ ( ${C}_{4}\le {C}_{1}$ and ${C}_{4}\le {C}_{2}$ ).
Table 2. Units of Friction Formulation I_{fric} Fric C_{1} C_{2} C_{3} C_{4} C_{5} C_{6} 1 $\left[\frac{1}{\text{Pa}}\right]$ $\left[\frac{\text{s}}{\text{m}}\right]$ $\left[\frac{\text{s}}{\text{Pa}\cdot \text{m}}\right]$ $\left[\frac{1}{{\text{Pa}}^{2}}\right]$ $\left[\frac{{\text{s}}^{2}}{{\text{m}}^{2}}\right]$ 2 $\left[\frac{\text{s}}{\text{m}}\right]$ $\left[\frac{\text{s}}{\text{m}}\right]$ $\left[\frac{\text{s}}{\text{m}}\right]$ 3 $\left[\frac{\text{m}}{\text{s}}\right]$ $\left[\frac{\text{m}}{\text{s}}\right]$  I_{fric} = 3 (Renard law):
 I_{fric} = 0 (Coulomb friction):
 Friction filteringIf I_{filtr} ≠ 0, the tangential forces are smoothed using a filter:
(7) $${F}_{t}=\alpha \cdot {{F}^{\prime}}_{t}+\left(1\alpha \right)\cdot {{{F}^{\prime}}_{t}}^{1}$$Where, α coefficient is calculated from: If I_{filtr} = 1: $\alpha ={X}_{freq}$ , simple numerical filter
 If I_{filtr} = 2: $\alpha =\frac{2\cdot \pi}{{X}_{freq}}$ , standard 3dB filter, with ${X}_{freq}=\frac{dt}{T}$ , and T = filtering period
 If I_{filtr} = 3: $\alpha =2\cdot \pi \cdot {X}_{\mathit{freq}}\cdot dt$ , standard 3dB filter, with X_{freq} = cutting frequency
The filtering coefficient X_{freq} should have a value between 0 and 1.
 Friction penalty formulation I_{form}:
 If I_{form} = 1 (default) viscous formulation, the friction
forces are:
(8) $${F}_{t}=\text{min}\left(\mu {F}_{n},{F}_{\text{adh}}\right)$$While an adhesion force is computed as:(9) $${F}_{\text{adh}}=C\cdot {V}_{t}\text{\hspace{0.17em}}\text{with}\text{\hspace{0.17em}}\text{\hspace{0.17em}}C={\mathit{VIS}}_{F}\cdot \sqrt{2\mathit{Km}}$$  If I_{form} = 2, stiffness formulation, the friction forces
are:
(10) $${F}_{t}^{\mathit{new}}=\text{min}\left(\mu {F}_{n},{F}_{\mathit{adh}}\right)$$While an adhesion is computed as:(11) $${F}_{\mathit{adh}}={F}_{t}^{\mathit{old}}+\mathrm{\text{\Delta}}{F}_{t}\text{\hspace{0.17em}}\text{with}\text{\hspace{0.17em}}\text{\hspace{0.17em}}\mathrm{\text{\Delta}}{F}_{t}=K\cdot {V}_{t}\cdot {\delta}_{t}$$Where, ${V}_{t}$ is the contact tangential velocity.
I_{form} = 2 is recommended for implicit and low speed impact explicit analysis.
 If I_{form} = 1 (default) viscous formulation, the friction
forces are:
 Orthotropic friction for shell
elements, if Idir = 1.
 Two sets of friction coefficients must be defined after the line that contains Idir
 The orthotropic directions are defined only on the main contact surface
 The 2 ways to define the orthotropic friction direction
 Use the orthotropic direction from the shell element as defined
in /PROP/TYPE9,
/PROP/TYPE10,
/PROP/TYPE11,
/PROP/TYPE17,
/PROP/TYPE51, or
/PROP/PCOMPP.
Direction 1 from element.
Direction 2 is orthogonal to Direction 1 in the segment plane.
 Use Direction 1 defined from the vector $V$ and angle $\varphi $ defined in /FRIC_ORIENT.
 Use the orthotropic direction from the shell element as defined
in /PROP/TYPE9,
/PROP/TYPE10,
/PROP/TYPE11,
/PROP/TYPE17,
/PROP/TYPE51, or
/PROP/PCOMPP.
 Not supported for solid element, beam, truss or spring elements or edge to edge contact.